1. Field of the Invention
The present invention relates to a color encoding video system utilizing an optical low pass filter and more particularly to an improved color encoding video system that is independent of the aperture opening of any ancillary image forming optical system utilized therewith.
2. Description of the Prior Art
In the field of single or double tube color television systems, a luminous signal and chromatic signals are derived from an object scene that has been imaged onto a color encoding filter by an objective lens system. The color encoding filter spatially modulates the image to generate respective chromatic signals for the separate colors that form the composite image. In general, the carrier frequency for the chromatic signals are dependent on the pitch of the stripes in the color encoding filter as measured along the scanning direction of an image pickup tube.
The prior art color encoding filters that are generally utilized can be classified into a two-color encoding and three-color encoding type. The accompanying electronic circuitry is capable of separating and processing the signals and is generally classified as either a frequency separating type or a phase separating type. In either case, when the image of the object scene includes a high-frequency component such as striped pattern having a spatial frequency close to that of color encoding filter, so that a high frequency component can be generated which would fall into a carrier frequency band of chromatic signals, then this high frequency component will interfere with the chromatic signals and produce false color signals for transmission and ultimately formation on a television screen. Numerous attempts have been made to efficiently eliminate such false chromatic signals for example, by placing an optical low pass filter in the optical system of the color television camera. The filter being capable of cutting off a high frequency component of the scene light before it impinges upon the photosensitive element of the image pickup tube.
Various examples of these optical low pass filters of a phase grating type can be found in the prior art such as U.S. Pat. No. 3,756,695 and Japanese Patent publication Nos. SHO49-20553 and SHO48-53472. Other examples of phase grating type optical low pass filters are disclosed in U.S. Pat. Nos. 3,911,479, 3,821,795, 3,681,519 and 2,733,291 and German publication DOS2,163,443.
The prior art has also utilized double refraction birefringent type low pass optical filters as disclosed in U.S. Pat. No. 3,588,224, German publication DOS2,027,035 and the Japanese utility model publication No. SHO47-18688.
Of general interest, is the disclosure in U.S. Pat. Nos. 3,875,627, 3,768,888, 3,566,016, 3,910,683 and 3,912,962.
Additionally, the prior art has attempted to use optical phase grating filters having various arrangement of their grating such as in an orderly alignment and in a random alignment. Many of these optical phase grating filters do have the capability of cutting off a high frequency component from the object scene as it is imaged through a particular optical lens system. However, these optical filters are generally mounted intricately in the optical lens system. As can be expected, the resulting cutoff frequency will be subjective to the particular type of color encoding system utilized in that color television camera. This subjectivity means that the objective lens system can only be used on a particular color encoding video system and if it is desirable for the objective lens system to be replaced, or to be used on another camera body requring a different cutofff frequency, then the optical low pass filter would not function properly. Thus, the prior art has frequently found that it is not possible to interchange objective lens systems on different camera bodies.
To date, the prior art has not provided a highly adaptable color encoding video system with an optical filter that is capable of removing optical aberrations within the constraints of an economical production of the optical filter and being independent of the objective lens system.